(CNN) -- The failed attempt to blow up a U.S.-bound passenger airplane on December 25 has raised questions about the need for the introduction of full-body scanners at all major airports.

The Netherlands said Wednesday it was installing the scanners at Amsterdam's Schiphol airport, where security staff failed to detect explosives being taken aboard Northwest Airlines flight 253.

Nigeria, where 23-year-old suspect Umar Farouk AbdulMutallab, began his journey, also said it would introduce the scanners.

How do these scanners work?

There are two types of device which offer full-body scans. Millimeter wave scanners use extremely high frequency radio waves which are processed by a computer to produce a detailed 3D image of air passengers. Backscatter scanners use high energy rays that -- unlike X-rays which penetrate objects -- scatter when they hit materials, allowing computers to render a detailed image and detect substances such as explosives and plastic weapons. Both scanners, unlike conventional X-rays, can strip away layers of clothing, accurately mapping the contours of the body, any prosthetics beneath the skin, as well as clothing and metallic and non-metallic objects.

How long does it take?

The scanning process takes between 15 and 30 seconds. Passengers enter a small booth or archway and raise their hands while radio waves target them from all directions. It may take slightly longer for airport staff to review the images produced and -- given the level of detail, more passengers may find themselves subjected to follow-up security checks as a result.

Why are they controversial?

Privacy campaigners say the scanners produce "naked" images of passengers which represent an unnecessary violation. They say the process is humiliating, and despite pledges that the images will not be stored or used elsewhere, it could be open to abuse -- particularly with scans of celebrities. Although the U.S. Transportation Security Agency, which is installing the scanners in many airports, insist the equipment does not capture details of face or produce images of a quality that can be deemed compromising, opponents say the technology is still capable of this and may be utilized in the future.

Who sees the images?

Current systems use two security officers. One works the machine and never sees the images, which appear on a computer screen behind closed doors. Another officer, who never sees the passenger, views the images. Software is being developed that could replace human operators with a computer, however there are concerns that electronic surveillance would fail to pick up on the same kind of detail.

Do they violate privacy laws?

Despite claims by some campaigners, neither U.S. nor European Union laws restrict the use of full-body scanners, although there has been some confusion over this. The European Commission -- the EU's executive arm -- is currently developing a consensus on the scanners, but final decisions remain with individual member nations.

Are there any other concerns?

Developers say the level of radiation used by backscatter scanners is so minimal there are no major health concerns, while the TSA says millimeter wave technology involves a fraction of the radio waves used in a cell phone.

Where have they been installed?

Full-body scanners are currently being trialed at Manchester Airport in England and Tokyo's Narita Airport in Japan. Amsterdam's Schiphol airport has 15 scanners, which are being drafted into active service, while Israel's Ben Guiron airport is already using them. In the United States, 40 scanners have been installed at 19 airports across the country. Six are used as the primary security check, while 34 are used for secondary, or random checks. More are expected to be installed.